Businesses Use Sensors to Track Customers, Build Shopper Profiles

Fan Zhang, the owner of Happy Child, a trendy Asian restaurant in downtown Toronto, knows that 170 of his customers went clubbing in November. He knows that 250 went to the gym that month, and that 216 came in from Yorkville, an upscale neighborhood.

And he gleans this information without his customers' knowledge, or ever asking them a single...